1. Field of the Invention
This invention relates to instrumentation amplifiers.
2. Description of the Related Art
Instrumentation amplifiers are designed to amplify the difference between two voltage inputs with a defined gain, and to produce a single-ended output referenced to a known reference point, usually ground. Implementing an instrumentation amplifier in a general purpose fashion is somewhat complex using conventional operational amplifiers (op amps). A classic three op amp implementation is shown in FIG. 1. A pair of input op amps A1 and A2 receive input signals V.sub.INP V.sub.INM at their respective non-inverting inputs, while their inverting inputs are tied together through a gain resistor Rg. Each op amp A1, A2 has a respective feedback resistor Rfb connected between its output and its inverting input. The outputs of A1 and A2 are connected through respective resistors R1 and R2 to the noninverting and inverting inputs of a third op amp A3, respectively, with another resistor R3 tapped off the noninverting input of A3 to a voltage reference V.sub.ref (normally ground potential). The circuit output is taken from the output of A3, which includes a feedback resistor R4 to its inverting input. Voltage supplies V+ and V- provide the required supply voltages to each of the op amps.
Assuming R1, R2, R3 and R4 are equal, the overall transfer function for the circuit of FIG. 1 is: EQU V.sub.o -V.sub.ref =(V.sub.INP -V.sub.INM)(2Rfb/Rg+1)
This transfer function is somewhat awkward from a resistor value viewpoint, and additionally can never have a gain of less than unity. Two other significant drawbacks of the circuit are that the common mode input range reduces in the presence of a differential signal, and R1-R4 all have to match extremely well for good common mode rejection; any trimming performed on these resistors interacts with the gain equation.
Another restriction associated with the conventional instrumentation amplifier circuit is in the area of supply voltages. The amplifier's output should be able to swing at least down to ground. However, because of the amplifier's output resistance, its output voltage cannot be brought down to ground unless the negative supply voltage is less than ground. This requirement is not a problem if there are two supply voltages available (V+ and V-). In some instances, such as with most disk drives, there is only a single voltage supply available (V+), and V- must be set at ground. The standard instrumentation amplifier is not applicable to these situations.